1. Field of the Invention
The invention relates to multilayer propylene polymer film which comprises at least one base layer comprising propylene polymer or a propylene polymer mixture and resin, and at least one outer layer comprising .alpha.-olefinic polymers having 2 to 10 carbon atoms.
The film according to the invention is distinguished by good twist behavior and very good migration properties.
The invention furthermore relates to a process for the production of the multilayer film and to the use of the film.
2. Description of the Related Art
A particular type of full wrapping closure for the purpose of packaging is twist wrapping. This packaging method is useful in the full wrapping of relatively small objects, and is particularly preferred for round or virtually round articles such as candy, bottles, candles, roll-packed confectionery, bars of chocolate and marzipan, Easter eggs or the like.
The prerequisite for use of the film in this area of application is its twist capacity. The resilience of the film must not result in it twisting back again after the pleat has been twisted, which means that the packaged goods can easily fall out. According to the known art, twist wrappings are made predominantly from cellophane, i.e., regenerated cellulose, un-oriented polypropylene or PVC film. For example, see "Verpacken mit Kunststoffen" [Packaging using Plastics] by Gunther Kuhne, published 1974, Carl-Hanser-Verlag, Munich, page 63.
Very recently, polypropylene films which have been biaxially oriented to different extents have been proposed for use in twist wrapping. These films contain a low-molecular-weight resin in order to achieve the desired twist properties. However, these known twist wrapping films have unsatisfactory migration behavior. It is known that certain additives which are added to the base layer of polypropylene films migrate to the surface after a certain time. This migration is desirable since the migrating additives develop an advantageous effect on the surface of the film.
DE-C-20 45 115 describes the use of hydrogenated polymer resins in unsupported polyolefin films, where the hydrogenated, vinyl aromatic hydrocarbon resins have a mean molecular weight distribution such that not more than about 15% of the polymer has a molecular weight (Mw) outside the range from 600 to 20,000. It is stated that the compatibility of the resin having an Mw of greater than 20,000 drops considerably and resin having an Mw of less than 600 makes the film tacky. Within the range from 600 to 20,000, the mean molecular weight is not crucial. The addition of resin imparts heat-sealing properties to the polyolefin. The polyolefin itself can only be heat-sealed with difficulty, or not at all.
GB-A-1,231,861 describes a biaxially oriented polypropylene (boPP) film which has good twist properties achieved by mixing the propylene homopolymer with a low-molecular-weight resin and by preferential orientation in the machine direction. Low-molecular-weight resins described are terpene polymers, compatible hydrogenated hydrocarbon resins or compatible rosins. The resins are characterized by their molecular weight, by their softening point and by their iodine number. Accordingly, preferred resins are those which have a molecular weight of greater than 600, a softening point of greater than 100.degree. C. and an iodine number of less than 15. An example mentioned of a typical resin is Picopale 100 having a molecular weight of about 1170, a softening point of about 100.degree. C. and an iodine number of 200.
The resins described are highly compatible with the propylene homopolymer and can contribute to good mechanical properties and very good twist properties.
The resin, which has a very low molecular weight compared with the propylene homopolymer, has considerable disadvantages with respect to its preparation, processing and use in polypropylene films. The resin tends to separate out and diffuse to the surface of the film. During the production process, this can result in an increased tendency of the film to stick. Deposition of the resin, such as, on rolls can also result. During processing, the packaged objects, such as candy, tend to block, which prevents or restricts their further transport.
Another serious disadvantage of the low-molecular-weight resins in the wrapping film occurs when the film is in contact with packaged contents, in particular with fat-containing contents. The resin migrates into the food after a short time, which is prohibited by food regulations. Films which contain migrating low-molecular-weight resins of this type therefore cannot be used for packaging fat-containing foods.
EP-A-0 217 388 describes a film which is also distinguished by good twist properties. In order to achieve the good twist behavior, a low-molecular-weight resin is added to the propylene homopolymer in the base layer. In addition, the process conditions are selected so that the modulus of elasticity in both orientation directions is greater than 3000 N/mm.sup.2. In the example, the resin mentioned is Arkon P 125, which has a molecular weight of about 1000 and a softening point of 125.degree. C. In addition to the above-mentioned advantages, this resin has significant disadvantages in the production of films, in the processing of films and on contact of the film with fat-containing foods.
Although the films described are covered with non-resin-containing outer layers, diffusion of the resin to the surface of the film still occurs even on contact with hot rolls (for example during longitudinal stretching) such that deposition of the resin on the roll surfaces can occur relatively quickly. The machine must be frequently cleaned, which results in significant loss in capacity.
Furthermore, these resins also have a very pronounced migration behavior. On contact of the film with fat, virtually all the resin present in the film has migrated into the fat, for example, after a test time of 10 days at a test temperature of 40.degree. C.
GB-A-2,055,688 describes a multilayer, heat-sealable, transparent biaxially-oriented polypropylene (boPP) film, which has good mechanical properties and good heat-sealing properties. In order to improve the mechanical properties, the base layer contains a low-molecular-weight resin in addition to propylene homopolymer. The resin content is from 2 to 20% by weight. The outer layers essentially comprise a mixture of at least two copolymers which give the film excellent heat-sealability. The film is biaxially stretched, using both stepwise stretching and simultaneous stretching. The latter type of stretching is expensive, but can result in good twist properties of the film. However, in the examples, the resin mentioned is Arkon P 125, which has a molecular weight of about 1000, and therefore has the above-described extreme disadvantages with respect to diffusion and migration behavior.
GB-A-2,028,168 describes a heat-sealable boPP film which has very good mechanical properties. In order to improve the mechanical properties, the base layer contains a low-molecular-weight hydrocarbon resin in addition to the propylene homopolymer. The resin content is from 1 to 50% by weight. The resin used has a molecular weight of greater than 600, preferably 1000, and a softening point of greater than 70.degree. C. Due to the relatively low molecular weight of the resin used, the above-described disadvantages are expected.
U.S. Pat. No. 4,921,749 describes a heat-sealable boPP film which has improved mechanical and optical properties. The heat-sealability of the film and the water vapor and oxygen permeability have likewise been improved. All the improvements result from the addition of a low-molecular-weight resin to the base layer. The resin content is between 3 and 30% by weight. The resin has a molecular weight of significantly less than 5000, preferably less than 1000, for example, 600. The softening point of the resin is from 120.degree. to 140.degree. C. Due to the stated stretching ratios of 5:1 in the machine direction and 10:1 in the transverse direction, the film has only limited suitability for twist wrapping. The low molecular weight of the resin means that the above described problems of diffusion of the resin through the film and migration into the food are very high.
EP-A-0 317 276 discloses a boPP film for twist application which contains a propylene homopolymer and a hydrocarbon resin in the base layer. The concentration of the resin is between 1 and 40% by weight. Due to the formulation chosen, the film is said to be distinguished by good dead-fold properties and to be suitable for use as a twist film. The resin used is, for example, a C.sub.9 -terpolymer (Arkon P 125), which has a molecular weight of about 1000. The disadvantages of a resin of this type were described above.